Underwater objects, particularly underwater objects that are in the water for long periods of time, have external surfaces that are subject to so-called “biofouling.” A used herein, the term “biofouling” is used to describe an attachment of organisms that live in the liquid, e.g., in the ocean, to surfaces, particularly to man-made surfaces. The organisms can be small, for example, algae, or larger, for example, barnacles.
Detrimental effects of biofouling to man-made surfaces are well known and wide-ranging. As is known, boats, ships, and other vessels that experience biofouling are subject to increased drag when operating in the water.
As is known, some types of coatings, for example, anti-biofouling paints, can be applied to some surfaces, for example, ship hulls, to prevent or retard biofouling. However, anti-biofouling coatings tend to degrade with time and need to be reapplied, for example, every few years. In order to reapply an anti-biofouling coating, a ship must be put to dry dock for the operation, resulting in high cost and ship down time.
Copper corrosion mechanisms or Tributyltin (TBT) biocide leaching are known. Electro-chlorination systems and automatic acid (e.g. tin dioxide) dispensing systems are also known. These mechanisms require release of chemicals into the water, proximate to the underwater surface, e.g., the ship hull. These mechanisms prevent biofouling on surfaces through localized production of bleach, via an oxidation of chloride ions present in seawater. Although the effects of such chemical systems are temporary, only lasting a few months, the effect on the environment is larger than desired for an anti-biofouling system. Furthermore the chemical release mechanisms are subjected to the ocean environment, e.g., pressure, resulting in reduced reliability.
Ultraviolet (UV) radiation consists of electromagnetic radiation between visible violet light and x-rays, and ranges in wavelength from about 400 nm to about 10 nm. UV is a component (less than 5%) of the sun's radiation and is also produced artificially by arc lamps, e.g., by a mercury arc lamp (or mercury vapor lamp).
Ultraviolet radiation in sunlight is often considered to be divided into three bands. Ultraviolet light in a UVA band (about 320-400 nm) can cause skin damage and may cause melanomatous (skin cancer). Ultraviolet light in a UVB band (about 280-320 nm) is stronger radiation that increases in the summer and is a common cause of sunburn and most common skin cancer. Ultraviolet light in a UVC band (below about 280 nm) is the strongest, having the greatest energy per photon (eV), and is potentially the most harmful form. Photon energy is calculated using: E=hv=hc/λ, where h is Plancks Constant, c is the speed of light, and λ is wavelength. Therefore, the lower the wavelength of electromagnetic radiation, the greater the energy per photon.
Much of the UVB radiation and most of the UVC radiation is absorbed by the ozone layer of the atmosphere before it can reach the earth's surface. Much of the UVB and UVC radiation that does pass through the ozone layer tends to be partially absorbed by ordinary window glass or by impurities in the air (e.g., water, dust, and smoke).
Ultraviolet germicidal irradiation (UVGI) is a sterilization method that uses specific UVC wavelengths (about 260 nm, e.g., 253.7 nm) to break down and kill microorganisms. Wavelengths of UVC radiation at or near 260 nm are known to be effective in destroying nucleic acids in the microorganisms so that their DNA is disrupted. Disruption of the DNA eliminates reproductive capabilities and kills the microorganisms.
U.S. Pat. No. 5,322,569, issued Jun. 21, 1994, describes an ultraviolet generating mechanism that can prevent biofouling underwater by way of a moving ultraviolet light source. U.S. patent application Ser. No. 12/862,084, filed Aug. 24, 2010 and entitled “Method And Apparatus For Anti-Biofouling Of Optics In Liquid Environments,” describes static UV light sources projected through an optics window.
It would be desirable to provide means to prevent biofouling of surface, e.g., an opaque surface, disposed in the water without using chemicals. It would be desirable to have such a system that can prevent biofouling to a degree that would reduce or eliminate the need to remove the surface, e.g., a surface upon a vessel, from the water.